It is well known in the multiple ratio transmission art that synchronizer mechanisms may be used to reduce shift time of all or some of the transmission gear ratios. It is also known that the shift effort required by a vehicle operator, i.e., force applied to a shift lever, may be reduced by use of synchronizer mechanisms of the self-energizing type. Since operator shift effort generally increases with vehicle size and weight, synchronizer mechanisms of the self-energizing type are especially important for heavy duty trucks. Prior art examples of such mechanisms may be seen by reference to U.S. Pat. Nos. 2,410,511; 2,896,760; 3,548,983; 4,413,715; 4,836,348; and 4,869,353 which are incorporated herein by reference.
The synchronizer mechanisms of the above patents include friction and jaw members for respectively synchronizing and positive clutching a gear to a shaft; blockers engaged in response to a pre-energizer effecting initial engagement of the friction members in response to initial engaging movement of one of the jaw members by a shift force, the blockers being operative to prevent asynchronous engagement of the jaw members and to transmit the shift force to the friction members to increase synchronizing torque thereof; and self-energizing ramps reacting the torque to provide an additional force in the direction of an additive to the shift force for further increasing the synchronizing torque of the friction members.
The basic purpose of such self-energizing synchronizer mechanisms is of course to provide faster synchronizing times with relatively moderate shift force from a manually operated shift lever for example. For a given synchronizer mechanism geometry and shift force, the additional force may be varied by varying the angles of the self-energizing ramps. In theory, ideal ramp angles exist which produce maximum controllable additional forces. For the frame of ramp angle references herein, the additional forces decreases as the ramp angles increase. Ramp angles less than the ideal angles produce uncontrollable additional forces, which once initiated, rapidly increase independent of the shift force and quickly drive the cone clutch to a lock-up condition. Ramp angles producing uncontrollable additional forces are said to be self-locking rather than self-energizing. Self-locking decreases shift quality or feel, may over stress synchronizer and other components, may cause overheating and rapid wear of the cone clutch surfaces, and may even override operator movement of the shift lever.
In practice the so called ideal ramp angles may vary substantially due to any of several variables, e.g., coefficient of friction variations, ramp surface wear, manufacturing tolerances, etc.